The present invention is based on Japanese Patent Application No. 2000-203661, each contents of which being incorporated by reference.
1. Field of the Invention
The present invention relates to a so-called tandem-type color image forming apparatus.
2. Description of the Related Art
As disclosed in U.S. Pat. No. 6,029,033, for example, in a conventional tandem-type color image forming apparatus, multiple image forming units that each house toner of a different color are aligned side by side above the intermediate transfer belt, toner images of different colors are formed on the image carrier, which comprises a photosensitive drum or similar component, of each image forming unit, and a multi-color combination image is formed by sequentially transferring onto the intermediate transfer belt each toner image from each image forming unit (primary transfer), and by then transferring the multi-color combination image on the intermediate transfer belt onto a sheet of recording paper (secondary transfer). Tandem-type image forming apparatuses of this type are receiving attention as color image forming apparatuses capable of high-speed printing.
On the other hand, Japanese Laid-Open Patent Application Hei 7-28294 discloses a tandem-type color image forming apparatus in which multiple image forming units that each house toner of a different color are aligned side by side below the intermediate transfer belt. In a tandem-type image forming apparatus in which the image forming units are located below the intermediate transfer belt in this fashion, the distance from the primary transfer position to the secondary transfer position, in regard to the image forming unit that is located furthest downstream in terms of the movement of the intermediate transfer belt, may be reduced relative to said distance in a tandem-type image forming apparatus in which the image forming units are aligned above the intermediate transfer belt. Consequently, an image forming apparatus in which the image forming units are located below the intermediate transfer belt offers the advantages that (1) the speed of first image formation may be increased. Such a construction when the image forming operation is stopped due to a problem such as a paper jam, (2) there are fewer toner images formed on the intermediate transfer belt, enabling the amount of wasted toner to be reduced, (3) it is easy to reduce the distance from the secondary transfer position to the fusing device, and (4) printing on small-size paper such as postcards may be performed.
In a tandem-type image forming apparatus of this type, the intermediate transfer belt is rotated using a driving roller. Single-color images are formed by each image forming unit on the intermediate transfer belt, and the combination image is then transferred onto a sheet of paper. The sheet of paper is conveyed to an area at which the combination image is transferred thereto via the rotation of the conveyance rollers. The conveyance rollers are driven to rotate together with the driving roller for the intermediate transfer belt. After the combination image is thermally fixed onto the sheet of paper, the sheet of paper is ejected onto a paper eject tray.
However, in a tandem-type image forming apparatus in which the image forming units are located below the intermediate transfer belt, because the load on the conveyance rollers changes from when a sheet of paper is being conveyed by the conveyance rollers rotating to when the sheet of paper has passed through them, the rate of rotation of the driving roller, which is driven by the same driving means as the conveyance rollers, fluctuates, leading to changes in the rate of rotation of the intermediate transfer belt. Where the rate of rotation of the intermediate transfer belt fluctuates, the problem arises that the single-color images formed on the intermediate transfer belt by the image forming units become disturbed, resulting in incomplete overlap of the images of the different colors.
An object of the present invention is to provide an image forming apparatus that, even when the rate of rotation of the intermediate transfer belt fluctuates when the sheet of paper passes through the conveyance rollers, does not cause misalignment of the single-color images formed on the intermediate transfer belt.
Another object of the present invention is to provide an image forming apparatus in which (1) the speed of first image formation may be increased, (2) the amount of toner wasted in the. event of cessation of an image forming operation due to a problem such as a paper jam may be reduced, and (3) printing on small-size sheets such as postcards is enabled.
The image forming apparatus of the present invention comprises:
an intermediate transfer belt being supported by a plural rollers including a driving roller and being driven to rotate by the driving roller;
a first image forming unit being located below the intermediate transfer belt, and forming a first toner image of a first color on the intermediate transfer belt at a first image forming position;
a second image forming unit being located below the intermediate transfer belt and being located downstream of the first image forming unit relative to the movement of the intermediate transfer belt, the second image forming unit forming a second toner image of a second color over the first toner image formed on the intermediate transfer belt at a second image forming position;
a third image forming unit being located below the intermediate transfer belt and being located downstream of the second image forming unit relative to the movement of the intermediate transfer belt, the third image forming unit forming a third toner image of a third color over the first and second toner images formed on the intermediate transfer belt at a third image forming position;
a fourth image forming unit being located below the intermediate transfer belt and being located downstream of the third image forming unit relative to the movement of the intermediate transfer belt, the fourth image forming unit forming a fourth toner image of a fourth color over the first, second and third toner images formed on the intermediate transfer belt at a fourth image forming position;
a transfer roller for transferring the superposed toner images formed on the intermediate transfer belt onto a sheet of paper in a transfer area, the transfer roller being disposed so as to press against the part of the intermediate transfer belt supported by the driving roller; and
a pair of rotatable conveyance rollers for conveying the paper to the transfer area in synchronization with the superposed toner images on the intermediate transfer belt,
wherein a first distance from the fourth image forming position for the fourth image forming unit to the transfer area is longer than a second distance from a nipping area formed by the pair of conveyance rollers to the transfer area.
In the image forming apparatus of the present invention, the sheet of paper is conveyed by the conveyance rollers such that the tip of the sheet arrives at the transfer area when the tip of the image formed by the fourth image forming unit reaches the transfer area. The rotational torque supplied by the roller driving device to the driving roller changes when the conveyance of the sheet by the conveyance rollers is completed, and at the same time the rate of rotation of the intermediate transfer belt also changes. Because the fourth image forming unit and the conveyance rollers are respectively located such that the first distance from the fourth image forming position to the transfer area is longer than the second distance from the nipping area formed by the conveyance rollers to the transfer area, when the sheet of paper passes through the conveyance rollers, the fourth image forming unit has completed image formation. Consequently, the effect of the fluctuation in the rate of rotation of the intermediate transfer belt is not reflected in the image transferred to the sheet of paper.
The first distance is slightly longer than the second distance, and the difference between the first and second distances is preferably approximately 10 to 20 mm. Based on this difference, when the sheet of paper is conveyed in synchronization with the combination image formed on the intermediate transfer belt, the sheet conveyance should begin with a slight time difference.
The fourth image forming unit preferably comprises an image forming unit that forms monochrome images. Because misalignment in the overlap of black images is more conspicuous than in images of different colors, this preferred location helps avoid such conspicuous misalignment.